A wave generator of a strain wave gearing causes a flexible externally toothed gear to flex into a non-circular shape and partially mesh with a rigid internally toothed gear, and causes the meshing position between the two gears to move circumferentially, whereby relative rotation between the two gears is generated in accordance with the difference in teeth therebetween. As proposed in patent documents 1 and 2, there is known a wave generator using piezoelectric elements which are solid displacement elements stretchable and contactable by energization.
The wave generator of this configuration flexes each part in the circumferential direction of an externally toothed gear repeatedly in the radial direction with constant amplitude and constant period, to thereby move the meshing position of the externally toothed gear with an internally toothed gear in the circumferential direction. Whereby, relative rotation according to the difference in teeth between the externally toothed gear and the internally toothed gear is generated between the two gears.